Snizhne (red marker) is strategically important, because it protects the crosspoint of roads (yellow) from the border to as well Lugansk as Donetsk (red)
The border post at Marinovka (blue) has been heavily contested the last week.

Of course it is a MAJOR development if Russian tanks are now in East-Ukraine and it seems the (pro)Russians are fortifying Snizhne now.

Difference is the T64B has the searchlight on its left side (for the viewers on the right). This is clearly the case, as well as in the video as in the photos.
So, ok it is a T-64B.

Are these (pro)Russians?
Without a doubt they are in possession of the (pro)Russians. They are roaming in an area where Ukraine is not operating. There is a border post Marinovka nearby that is desperately asking for assistance and reinforcement, but they are not getting any. Ukrainians tend to proudly carry Ukrainian flags with them. These don't have flags.
So definitely these are Ukrainian.

To sum up:
- there are tanks in (pro)Russian hands now
- Ukrainian government and Inforesist say they came across the border
- tank crew say they came across the border
- rebels say they were stolen
- so far there are no known reports about stolen tanks.

What might be possible is that this is Russia's way of 'returning tanks from Crimea' back to Ukraine?